Ocean Mixing Unveiled: How Waves and Turbulence Shape Our Seas
"New research sheds light on the complex interaction of waves and turbulence in the ocean mixed layer, offering critical insights for climate modeling and marine ecosystems."
The ocean is a dynamic environment where energy is constantly exchanged between the atmosphere and the water. This exchange is largely governed by turbulence, which mixes water, distributes heat, and influences marine life. Understanding how turbulence behaves in the ocean mixed layer (OML) is crucial for accurate climate modeling and predicting the health of our oceans.
Turbulence in the OML is a complex interplay of energy production, buoyancy (the tendency of an object to float), and dissipation (the conversion of kinetic energy into heat). Large-scale numerical models rely on accurate parameterizations of these processes, especially dissipation, to compensate for their inability to directly simulate turbulence at the smallest scales.
Recent research has focused on refining our understanding of how energy dissipates in the OML. Unlike the atmospheric boundary layer (ABL), the OML is significantly influenced by surface gravity waves and Langmuir circulations, creating more complex patterns of dissipation. This article delves into a groundbreaking study that examines these processes, providing new insights into wave-turbulence scaling in the ocean.
The Scale of Wave-Driven Mixing
Ocean turbulent mixing influences exchanges of heat, momentum, and mass between the air and sea. Surface gravity waves play a key role in generating turbulence in the upper ocean, but how wave energy transfers into ocean turbulence remains an open question. A 2025 estimate using an empirical scheme and wave reanalysis data placed total nonbreaking wave-energy dissipation through wave-turbulence interaction at approximately 17 TW globally, predominantly in extratropical regions during local winter. Combined wave-current boundary layers can also generate strong turbulence and high bed shear stresses, affecting sediment transport, mixing, and diffusion.
What Drives Turbulence in the Ocean's Surface?
Traditionally, scientists have approached turbulence in the OML using similarity scaling, which treats the OML as a shear-driven wall layer. This approach assumes a constant stress with a logarithmic velocity profile, where shear (the change in velocity with depth) is proportional to the friction velocity (a measure of wind stress) and inversely proportional to depth.
- Breaking Waves: Introduce significant turbulence near the surface.
- Langmuir Circulations: Enhance mixing and dissipation.
- Wind Forcing: Creates shear and contributes to turbulence production.
Refining Surface-Wave Mixing
Research on upper-ocean mixing examines how surface gravity waves transfer energy into shear turbulence, including regimes characterized by La values from 0.5 to 2. The FuGas 2.5 update reports that larger gravity waves are determinant for the roughness length in coastal oceans, while capillary-gravity waves and smooth airflow are also important. Measurements across the air-sea interface corroborate an input-dissipation balance for waves at wind speeds of 8-14 m s−1 in developed to young seas. Observations of turbulent dynamics further indicate that evaluating dissipation at only one depth may be insufficient for understanding the ocean surface boundary layer.
Why Simple Wave Models Fall Short
Earlier research often focused on wave-mean-current interactions and largely ignored wave-turbulence interactions because waves were mistakenly treated as principally irrotational. Subsequent work presented evidence of clear coupling between surface waves and surface turbulence, indicating that molecular layers, viscous layers, waves, and turbulence all matter for parameterizing air-sea fluxes. Another study concluded that wave-turbulence interaction can generate turbulence kinetic energy in the upper ocean and influence both vertical mixing and surface-wave decay. The continuing study of turbulent vortices in combined wave-current boundary layers shows that the relevant dynamics remain unresolved.
Testing Turbulence Profiles
An atmospheric-boundary-layer similarity approach calculates profiles of turbulent kinetic energy and energy dissipation rate. The reported profiles are required to match observed values at 5 and 40 meters. Its algorithms are coded for operational use and produce plausible profiles over the diurnal variation of the atmospheric boundary layer. The study therefore emphasizes comparison with observations at multiple heights rather than relying on a single-depth assessment.
The Future of Ocean Turbulence Research
While significant progress has been made, accurately parameterizing turbulence in the OML remains a challenge. The intermittent nature of turbulence and the scarcity of comprehensive datasets, particularly in the open ocean, limit the ability to validate and refine existing models. Future research needs to focus on collecting more high-quality data under diverse sea states and conditions to better understand the complex interplay of factors that govern turbulence in the ocean's surface.
From Measurements to Mechanisms
Analysis of measurements from the Knorr11 cruise found that the LOW predicts air-sea gas fluxes within an order of magnitude, but tends to overestimate the observations. Direct numerical simulations published in 2025 investigated turbulence induced by nonbreaking surface waves using a sigma-coordinate, free-surface nonhydrostatic model. One simulation was modeled on a laboratory experiment involving a wave with a 30 cm wavelength and 1 cm amplitude. Together, these studies illustrate the value of comparing observations with both parameterizations and mechanistic simulations.
Connecting Scales
Current research in ocean turbulence and mixing dynamics is expanding through new methods and studies from researchers across the field. The 12th Waves and Turbulence Dynamics workshop summarized state-of-the-art knowledge about waves and turbulence in geophysical flows, with particular focus on the ocean. Its cross-scale perspective addresses wave and turbulence generation, evolution, and interactions. These interactions govern energy transfer across scales and between processes, as well as dissipation that contributes to ocean mixing and the transport of heat, salt, and nutrients.
Improving Climate Models
As turbulent-mixing models become more sophisticated, their parameterizations can be evaluated by comparing modeled three-dimensional spatial patterns of diffusivity or dissipation with observations, including their temporal variations. Research on wave-turbulence interaction also aims to clarify how turbulence associated with nonbreaking waves contributes to vertical mixing. A stated goal is to develop a simple parameter that can represent these effects in large-scale geophysical applications, including general circulation and coupled climate models. The broader challenge is to connect detailed wave-turbulence processes with model representations that remain useful at climate scales.